Crust and upper mantle structure of the Ligurian Sea revealed by ambient noise tomography and receiver function analysis

The Liguro-Provencal-basin was formed as a back-arc basin of the retreating Calabrian-Apennines subduction zone during the Oligocene and Miocene. The resulting rotation of the Corsica-Sardinia block is associated with rifting, shaping the Ligurian Sea. It is debated though, whether oceanic or atypical oceanic crust was formed or if the crust is continental and experienced extreme thinning during the opening of the basin. We contribute to the debate by surveying the type of crust and lithosphere flooring the Ligurian Sea using 29 broadband Ocean Bottom Seismometer (OBS). The instruments were installed in the Ligurian Sea for eight months between June 2017 and February 2018, as part of the AlpArray seismic network. Because of additional noise sources in the ocean, OBS data are rarely used for ambient noise studies. However, we extensively pre-process the data, including corrections for instrument tilt and seafloor compliance, to improve the signal-to-noise ratio. We calculate daily cross-correlation functions for the LOBSTER array and surrounding land stations. Additionally, we correlate short time windows that include strong earthquake events that allow us to derive surface wave group velocities for longer periods than using ambient noise only. Group velocity maps are obtained by inverting Green’s functions derived from the cross-correlation of ambient noise and teleseismic events, respectively. Our group velocity maps show strong heterogeneities for short periods (5-15 s, corresponding to shallow depths). In general, the velocities increase with depth and the velocity anomalies can be related to varying sediment thickness and magmatism. The longer periods (20-90 s) show a smoother velocity structure that reveals mantle velocities in the vicinity of the Ligurian margin, north of the basin centre. However, resolution at greater depth is limited along the Corsican margin due to less station coverage. Our results do not indicate an oceanic spreading centre, however, may hint to an asymmetric opening of the Ligurian Basin.